Polyolefin microporous films are used as microfiltration membranes, separators for batteries, separators for condensers, materials for fuel cells, etc. Among these applications, the use of the microporous films as separators for batteries, particularly separators for lithium-ion batteries, requires that the polyolefin microporous films are excellent in ionic permeability, mechanical strength and the like.
In addition, separators for recent high capacity batteries have been required to have a “fuse effect”, “film-rupture resistance” and “low heat shrinkability” to secure the safety of the batteries and to be less variable in film thickness to reduce the variation of battery characteristics.
The “fuse effect” refers to a mechanism in which, when the inside of a battery is overheated during overcharge or the like, the separator melts, forms a coat covering an electrode and interrupts the current to secure the safety of the battery. For a polyethylene microporous film, the fuse temperature, i.e., the temperature at which the fuse effect is exhibited, is known to be roughly in the neighborhood of 140° C. However, a lower fuse temperature is considered to be better to stop a thermal runaway reaction and the like in the battery as soon as possible.
The “film-rupture resistance” refers to the performance of the separator, in which film breakage is prevented even when the separator is heated to not less than the fuse temperature. In addition, the “low heat shrinkability” refers to the performance thereof, in which heat shrinkage occurs only slightly even when the separator is heated to not less than the fuse temperature. Both performances are considered to be necessary for maintaining the shape thereof even after melting to keep the insulation between the electrodes.
Separators for batteries are each required to have a performance satisfying the safety evaluation criteria for batteries as prescribed in “Standard for Lithium Batteries” of U.S. standard UL1642 to assure the battery safety at 150° C. The evaluation is carried out by keeping a separator in an oven at 150° C. for 10 minutes. To achieve the criteria, it is desired that the separator is made non-porous by fusing at 130° C. to 140° C. and is not broken with minimal heat shrinkage even when heated at 150° C. or higher, thereby maintaining the shape thereof.
The “a small variation in film thickness” is an important performance for stably obtaining the above battery performance. The variation in film thickness of the separator has sometimes made it impossible to stably achieve the above fuse effect, film-rupture resistance and low heat shrinkability. The change of the distance between the positive and negative electrode plates also leads to the variation of the performance of individual batteries; for example, the use of the film causing the changed distance in pack batteries in notebook computer applications has further increased the variation in some cases, resulting in a reduction in yield. The film formation itself has also sometimes been difficult, leading to a decrease in yield.
Known techniques in which the film-rupture resistance of polyolefin microporous films has been improved include, for example, those described in Patent Documents 1 and 2.
Patent Document 1 discloses a polyolefin microporous film whose shape retention during melting is improved by blending polypropylene, which has a melting point higher than that of polyethylene. However, in the Example using the technique, a quality disadvantage has been found that particles are left in the resultant polyolefin microporous film because of the blending of a polyolefin composition containing a polyethylene having a weight average molecular weight of more than 2,500,000 with a polypropylene having a weight average molecular weight of the order of 500,000.
Patent Document 2 discloses a polyolefin microporous film characterized in that it contains as essential components a polyethylene having a certain viscosity average molecular weight and 7 to 50 wt % of polypropylene and in that the correlation between the molecular weight and terminal methyl group concentration determined using GPC/FTIR satisfies a particular relationship. However, this invention is intended to improve the oxidation resistance of the separator; it is hard to say that the film thickness uniformity and the lower heat shrinkage of the separator have been sufficiently achieved.
In addition, techniques in which it has been tried to uniformize the thickness of a polyolefin microporous film while improving the film-rupture resistance include that described in patent document 3. This document discloses a polyolefin microporous film containing a polypropylene having a weight average molecular weight of 500,000 or more and a melting heat of 90 J/g or more as measured by differential scanning calorimetry. The microporous film is described to feature excellence in film thickness uniformity. In the invention, the use of the polypropylene having a melting heat of 90 J/g or more can be interpreted to make excellent the dispersibility of polypropylene, thereby enabling the improvement of the film thickness distribution. However, blending polyethylene and polypropylene broadens the film thickness distribution owing to reduced melt viscosity, which has not been improved yet. In addition, it is described in the Examples that the standard deviation of the film thickness distribution (film thickness variation) achieved in the present invention was 1.7 to 2.8; however, these values have been unsatisfactory for improvement of the film thickness distribution.
With respect to low heat shrinkability, a method such as that described in patent document 4 is known which involves subjecting the separator to heat setting and thereby reducing the heat shrinkage. However, although the heat setting of the separator can reduce heat shrinkage at up to around the melting point of polyethylene, it has had a problem that the separator thermally shrinks inside the battery when reaching the temperature of 150° C. required in the safety criteria, which results in the short circuit between the electrodes. Thus, there has been a need for development of a separator having lower heat shrinkability.
It is also hard to say that the separators obtained in the above patent documents 1 to 4 have fuse effects improved compared to before; the characteristics cannot be said to have reached a satisfactory level.
As a technique for improving the film-rupture resistance, the present inventor discloses, in patent document 5, a polyethylene microporous film comprising a high-density or linear copolymer polyethylene having a terminal vinyl group concentration of 2 or more/10,000 carbon atoms in the polyethylene as determined by infrared spectroscopy. The microporous film has good film-rupture resistance, which has, however, been less than sufficient yet. Neither indication nor disclosure of a method for thinning the microporous film and narrowing the film thickness distribution is also not given.
Thus, it has represented a challenge to develop a separator combining improved film-rupture resistance, a good fuse effect and low heat shrinkability and also having uniform film thickness.    [Patent Document 1] Japanese Patent No. 3699561    [Patent Document 2] JP-A-2005-200578    [Patent Document 3] JP-A-2004-196870    [Patent Document 4] JP-A-09-012756    [Patent Document 5] Japanese Patent No. 3177744